Exposure to excessive sunlight is an important etiologic factor in the development of acute inflammation, which is characterized by erythema and edema. The long-term consequences of such inflammation include accelerated skin aging and a higher chance of developing skin cancer. Skin inflammation due to acute exposure to UV radiation has been shown to be characterized by the release of various factors including neuropeptides, histamine, prostaglandins, serotonin and oxygen radicals, as well as the upregulation of pro-inflammatory cytokines such as IL-1, IL-6 and tumor necrosis factor alpha (TNF-α). Skin epithelial cells and keratinocytes play an important role in the inflammatory processes observed in skin after UV exposure by producing several of the above factors.
The long term effects of UV-induced inflammation negatively alter skin function. Wound healing processes in skin that has suffered multiple episodes of inflammation are extended in time and can be imperfect (e.g., increased scarring). Furthermore, over-exposed skin is also more prone to wrinkling, dryness, thinning, sagging and greater susceptibility to bruising. The inflammatory processes in skin that lead to these negative effects are complex and likely involve several pathways.
UV light consists of both UVA and UVB rays. UVB is a well known cause of acute inflammation as well as non-melanoma skin cancer. UVB-mediated epidermal inflammation is orchestrated by pro-inflammatory cytokines such as IL-1, IL-6, IL-8 and TNF-α. Since UVB rays penetrate through the epidermis, cytokine induction can occur in keratinocytes, as well as in fibroblasts and endothelial cells that reside in the upper dermis. However, these latter two cell types, which are of mesenchymal origin, can also be induced to an inflammatory state in a manner somewhat indirect from direct UV exposure. Factors (i.e., mediators) expressed by epithelial cells and keratinocytes in response to UV stimulation can signal fibroblasts and endothelial cells to upregulate inflammatory pathways. IL-6 is one such mediator.
The induction of the cytokines such as IL-6 in epithelial cells and fibroblasts has a significant effect on the acceleration of skin photoaging, which manifests itself through wrinkling and sagging, amongst other indications. These effects are primarily mediated by the stimulation of excess MMP-1 expression by fibroblasts in response to IL-6 signaling (Fagot et al. Arch Dermatol Res. 293:576, 2002; Fagot et al. Photochem Photobiol. 79:499, 2004). MMP-1 overexpression can significantly alter the structural integrity of skin by degrading extracellular matrix (ECM) proteins such as collagen that are comprised in the skin's connective tissue. Furthermore, ECM breakdown enhances the recruitment of immune cells to the site of UV exposure; this heightened cellular activity is a main cause of acute symptoms such as erythema and edema, as well as chronic symptoms such as skin hardening, which results from excess fibrin deposition. Given these far-reaching negative effects on skin physiology in response to UV exposure, IL-6 and MMP-1 represent important molecular targets for controlling skin photoaging.
One means for controlling photoaging is the topical administration of proteins known to inhibit one or more signaling pathways that exhibit altered activity after UV exposure. However, most attempts employing such a strategy have failed to achieve clinically significant results, due in part to difficulties associated with use of entire proteins or large fragments thereof. One problem underlying this failure relates to the inefficient delivery of proteins across the epidermis; most of the applied protein remains distant from the cells that are responsible to initiating photoaging pathways. Other drawbacks relate to the high lability and poor retention of large proteins after administration. Aside from these inherent negative features, the development of these therapies also suffers from the complexity and high costs associated with preparing large proteins. Therefore, less expensive and more effective preparations are presently sought.
Short bio-active peptides represent a potentially useful means for treating and preventing skin photoaging. Besides the immediate benefits of being less expensive and more easily produced and manipulated, short peptides are also better absorbed and retained by skin. Regarding the prevention of photoaging in skin, short peptides (e.g., tetrapeptides) capable of inhibiting skin inflammatory processes are desired.
Although others have previously tested the effects of tetrapeptides on skin, few have been shown to inhibit the inflammatory processes known to be upregulated in skin by UV radiation. For example, Lintner (U.S. Pat. No. 6,974,799) employed certain tetrapeptide-tripeptide mixes to allegedly reverse aging signs in skin; however, this mix was only shown to upregulate ECM production, a process which would not be predicted to prevent the deleterious effects of sunlight. In a similar vein, Sandberg et al. (U.S. Pat. No. 6,962,904) teaches the use of elastin-derived tetrapeptides to restore connective tissue in skin. Particular tetrapeptides are purported by Bissett et al. (U.S. Pat. No. 6,284,802) to be useful for treating wrinkles; the only basis for this utility is the derivation of the peptide from the amino acid sequence of basic fibroblast growth factor (bFGF). As such, these peptides may not be expected to exert anti-inflammatory activity, since bFGF is known to play a positive role in immune cell recruitment (Zittermann and Issekutz Am J Pathol. 168:835, 2006). Tetrapeptides described by Dussourd et al. (U.S. Pat. No. 6,211,155) to stimulate epidermal cell proliferation are also not expected to inhibit UV-induced inflammation. On the other hand, the instant invention provides tetrapeptides that down-regulate UV-induced inflammation in skin, therefore acting to prevent or treat the main etiology of photoaging.